1. Field of the Invention
The present invention relates to a temperature sensor and more particularly to an improved connection between a terminal and a lead extending from a temperature sensing element.
2. Prior Art
FIG. 10 shows a conventional temperature sensor 1 used for measuring the temperature of cooling water of an automobile. The temperature sensor 1 has a holder 2 made of brass and is dipped into the cooling water of the automobile flowing in the direction of arrow 11. The holder 2 has an elongated protection tube 2a downwardly extending to a bottom of the holder 2. The holder 2 has on an upward end thereof a radially projecting hexagonal flange 2b to which a mounting tool fits when the temperature sensor 1 is mounted to an engine. The holder 2 has a threaded portion 2c below the hexagonal flange 2b, so that the holder 1 is mounted by screwing into a cylinder wall 12 of the engine. The protection tube 2a houses a temperature sensing element 3 in the form of thermistor connected to a lead 4. The temperature sensing element 3 and the lead 4 are embedded in a protective resin filled into the holder 2. The protective resin 5 forms a connector body 6 upwardly extending from the opening of the holder 2. A terminal 7 is connected with the end portion of lead 4 and projects into a socket receiving recess 6a. The connection at which the terminal 7 is connected with the lead 4 is also embedded in the protective resin 5. The terminal 7 is an elongated plate like terminal, as shown in FIG. 11, which is gold-plated for good conduction of a very small current. The lead 4 is soldered at 8 to the surface of terminal 7. A tube 9 covers the lead 4 to protect and insulate it from surroundings. A cap 10 serves as a simple water-proofing material for the temperature sensing element 3.
The prior art temperature sensor 1 of the aforementioned construction operates to determine the change in the temperature of cooling water in terms of the change in resistance of the temperature sensing element 3 firmly supported in the protective tube 2a. Generally, very small electric power is used when measuring the temperature, so that heat generation due to the current through the thermistor is as low as possible. In fact, the current through the thermistor is in the range from 100 uA to 2 mA. The temperature sensor 1 is capable of measuring as wide a temperature range as -40 to +130 degrees.
With the prior art temperature sensor, the linear expansion of leads 4 due to elevated temperatures is different from that of resin 5 that holds the temperature sensing element 3 and the leads 4, resulting in the differences in repetitive tensile forces exerted on the lead during heat cycle. The tensile forces are exerted as mechanical loads to the solder 8, which forces tend to pull the leads 4 off the terminal 7. Consequently, the soldered portion gradually loses the effect of securing the leads to the terminal 7 in the long run. The resultant poor soldering effect results in a poor electrical contact between the lead 4 and terminal 7.